We report here on a sequence of 28 observations of the binary pulsar system PSR B1259−63/SS2883 at four radio frequencies made with the Australia Telescope Compact Array around the time of the 2000 periastron passage. Observations made on 2000 September 1 show that the apparent rotation measure (RM) of the pulsar reached a maximum of −14800 ± 1800 rad m−2 , some 700 times the value measured away from periastron, and is the largest astrophysical RM measured. This value, combined with the dispersion measure, implies a magnetic field in the wind of the Be star of 6 mG. We find that the light curve of the unpulsed emission is similar to that obtained during the 1997 periastron but that differences in detail imply that the emission disc of the Be star is thicker and/or of higher density. The behaviour of the light curve at late times is best modelled by the adiabatic expansion of a synchrotron bubble formed in the pulsar/disc interaction. The expansion rate of the bubble ∼12 km s−1 is surprisingly low but the derived magnetic field of 1.6 G close to that expected.